This invention relates to radio antennas, and particularly to high-frequency, parallel-element, phased array antennas.
In the commercial and amateur, high frequency radio communications art the size and weight of an antenna are often as important, or nearly as important, as its electromagnetic characteristics. This is because, particularly at high frequency wavelengths, the physical dimensions of an antenna may dictate whether it can be used at a particular location and can have a significant impact on the cost of the antenna and its installation. In general, for given gain, directivity, bandwidth and impedance matching characteristics, it is desirable to make an antenna as compact and lightweight as possible. In addition, the electrical characteristics of an antenna are affected by the in situ environment of the antenna, so it must be tuned and its input impedance must be adjusted to account for that environment.
One well known type of antenna that provides high gain and directivity is the planar, parallel-element, phased array antenna. In such an antenna, two or more elongate conductive elements are disposed parallel to one another in the same plane, spaced apart from one another by selected amounts to form an array, and supported by a boom disposed perpendicular to the array elements. The gain and directivity of such antennas is determined primarily by the number of elements, the spacing between the elements and the relative phases of the currents in the elements. One or more of the elements is connected directly to the radio, and others may be coupled indirectly to the radio by electromagnetic field interaction among the elements. Where all the elements of the array are connected directly to the radio the antenna is known as xe2x80x9cdriven array.xe2x80x9d (It is well understood in the art that an antenna generally has the same electromagnetic characteristics whether it is xe2x80x9cdrivenxe2x80x9d by a radio transmitter or connected to a receiver to receive electromagnetic radiation.) Where not all of the elements of an array are connected directly to the radio, that is, not all elements are xe2x80x9cdriven,xe2x80x9d the antenna is known as a xe2x80x9cparasitic array,xe2x80x9d the elements that are connected being referred to as driven, and the elements that are not connected being referred to as parasitic.
The commonly known three-element parasitic phased array, generally known as a three-element xe2x80x9cYagi,xe2x80x9d can provide excellent gain, directivity and bandwidth characteristics at high frequencies, but cannot be made very compact or lightweight. The more compact and lightweight two-element parasitic array can achieve most of the gain of a three element parasitic array which has been optimized for directivity, but the two-element Yagi cannot simultaneously provide adequate front-to-rear power ratio.
The two element driven array is well known in both the amateur and the commercial radio communications art. It comprises two parallel conductive elements which are spaced a selected distance apart and both of which are connected directly to the radio, usually with a transmission line that presents a different phase to one element than the other element. In particular, there have been a number of popular variants of the two-element driven array in common use by the amateur radio community. These include antennas known as the W8JK array, and various embodiments of unidirectional designs known as the xe2x80x9cHB9CV arrayxe2x80x9d or xe2x80x9cZL Specialxe2x80x9d.
All of the aforementioned two-element driven arrays have their strengths and weaknesses. For example, the W8JK array, which uses two equal length elements fed 180 degrees out of phase, is easy to construct and capable of providing a significant amount of bidirectional gain. However, in order to provide a large amount of gain, the elements must be closely spaced, and when spaced as closely as about 0.125 wavelengths or less, the radiation resistance falls precipitously for both elements. As a result, losses become significant. Also, since the antenna is bidirectional, it is less useful for interference rejection than a unidirectional design.
The aforementioned HB9CV array alleviates both the loss and front-to-back power ratio problems of the W8JK antenna by using elements which are not the same length and by operating them at a relative phase angle other than 180 degrees. All known variations of this design are believed to use stagger tuned elements spaced at about 0.125 wavelengths and differ in the phasing and feed methods that are used. An important drawback, however, is that the tuning and input impedance matching of such antennas cannot be adjusted in situ; rather, the antenna must be removed from its in situ support structure, typically a mast, physically adjusted, and then put back in place. This is not only physically awkward, but it often prevents the antenna from being optimally adjusted since the required adjustment is affected by the real environment in which the antenna operates.
Accordingly, there is a need for a relatively compact unidirectional antenna having good front-to-rear performance, high efficiency and reasonable gain whose optimal resonance frequency and input impedance match can be adjusted in situ.
It has been found that a novel two-element driven array according to the invention can provide at least as much gain as a two-element Yagi together with front-to-rear directivity comparable to that which is available from a three-element Yagi. It has less than half the boom length and only about two thirds the mass of a three element Yagi of similar electromagnetic properties. The novel two-element driven array can be tuned in situ and its input impedance can be adjusted in situ.
To that end, the invention provides an array antenna comprising a first elongate element having a feed point gap disposed substantially at the center thereof, a second elongate element having a feed point gap disposed substantially at the center thereof, the second elongate element being substantially coplanar and parallel with the first elongate element and spaced a predetermined distance therefrom; a phasing transmission line connected at a first end thereof to the feed point gap of the first elongate element and at the second end thereof to the feed point gap of the second elongate element; and a tuning transmission line connected at one end thereof to the second end of the phasing transmission line. The first end of the phasing transmission line is the feed point for the antenna.
The transmission line connected to the second end of the phasing transmission line preferably comprises a variable transmission line having a movable member for shorting one side of the transmission line to the other side thereof for tuning the resonant frequency of the antenna. The phasing transmission line preferably comprises a pair of unbalanced coaxial cables tucked into the boom for protection. A combined balun and impedance matching network is preferable provided for matching the antenna to an unbalanced, coaxial feed line. Combined network preferably includes a shorting stub to adjust the impedance match between the antenna and the feed line.
The two elongate elements are mounted on a boom perpendicular thereto. The tuning transmission line and combined balun and impedance matching network may be folded back on the boom for ease of access in situ.
These features provide an optimum combination of antenna gain, directivity, bandwidth and impedance matching in a compact, lightweight, two-element driven array, together with in situ tuning and impedance match adjustment.
The foregoing and other objects, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.